vapor intrusion attenuation factors (afs) – measured vs . epa defaults a case study

Gradient CORPORATION

Vapor Intrusion Attenuation Factors (AFs) – Measured vs. EPA Defaults

A Case Study

Presented byManu Sharma and Jennifer DeAscentis

Presented at EPA Vapor Intrusion Workshop

Amherst, Massachusetts

October 18, 2004


Outline

Site Setting

Environmental Data

Measured and Default AFs

Role of Background

Modifications to Default AF


Site Setting

Active equipment testing facility located in the northeast US

Urban setting – highway; other industrial/ commercial operations nearby

Long history of industrial activities – low volumes of chlorinated organics and hydrocarbon usage

Glacial outwash deposits – relatively high hydraulic conductivity

Depth to groundwater ~ 6 meters


Outline

Site Setting

Environmental Data


Role of Background



Sampling Locations


Environmental Data

Sources and groundwater plume reasonably defined

Low VOC concentrations present in groundwater – order of 1 to 100 ug/l

Indoor air sampling locations

• Known sources of VOCs (e.g., historical solvent usage areas)

• Small and large buildings with different ventilation characteristics


Indoor Air and Groundwater Data

Indoor Air Background

(ug/m3) aIndoor Air Concentrations

(ug/m3) b

Indoor Air Sample #1

Trichloroethene 0.15-9.01 5.8 20 - 120


Tetrachloroethene 0.29-27.8 0.39-0.41Trichloroethene 0.15-9.01 0.63


Benzene 2.14-16.8 1.2-1.9

Trichloroethane, 1,1,1- NA g 0.49


Benzene 2.14-16.8 0.59-4.5

Dichlorodifluoromethane NA g 3.6-7.9 49 - 60Ethylbenzene 5.08-14.3 0.3-3.1Tetrachloroethene 0.29-27.8 0.51-4.2 1.2 - 1.6Toluene 26.9-80.0 1.1-14 1 - 49

Trichloroethane, 1,1,1- NA g 2.9-150Xylenes 20.0-24.2 0.72-11 2.1 - 220

1.3

5.8

96

25

26

22

Groundwater

Concentrations (ug/L) b

5


Outline

Site Setting

Environmental Data


Role of Background



AF Calculation

Measured AFs

• Range: average air concentration and range of measured groundwater concentrations at associated wells

• Recommended Estimate: average air concentration and average groundwater concentration (temporal and spatial) at associated wells

Default AF from Figure 3b of EPA guidance

• Depth to contamination = 4 m

• Sandy soils


Measured vs. EPA Default AFsFigure 3: Measured Site Specific AFs by Constituent versus USEPA-recommended Semi-Site Specific AF

2.7E-04

1.9E-04

2.7E-05

1.6E-05

5.3E-05

1.4E-03

2.8E-05

1.9E-04

6.9E-04

4.5E-043.6E-04 3.9E-04

1.1E-04

2.3E-05

5.5E-03

6.5E-02

2.2E-02

2.0E-03

2.6E-05

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

Mea

sure

d S

ite

Sp

ecif

ic A

tten

tuat

ion

Fac

tor

(Min

imu

m, M

axim

um

)

USEPA Semi-Site Specific Attenuation Factor (1.0E-03)

Benzene Dichlorodifluoro-methane

Ethyl-benzene

Tetrachloro-ethene

Toluene Trichloro-ethane, 1,1,1-

Trichloro-ethene

Xylenes

n=2

n=3

n=1

n=3

n=2

n=2

n=4

n=3

(AS-3)

(AS-4)

(AS-4)

(AS-4)

(AS-4) (AS-4)

(AS-4)

(AS-2)

(AS-1)

(AS-1)

(AS-3)

Note:AF = Attenuation FactorHighlighted attenuation factor indicates low levels of constituent detected in groundwater (i.e., equal to or less than 5 µg/L, the typical Method Detection Limit (MDL) for VOCs)

– - recommended AF by constituent and air sampling location


Measured vs. EPA Default AFs (Cont’d)

Measured AF about an order of magnitude less than default AF (10-4 vs. 10-3)

True AF may be even lower because:

• Air sampling locations near sources – air concentrations biased high

• Monitoring wells located downgradient of sources – groundwater concentrations biased low

No trend in measured AFs vs. building size or ventilation noted

Background may be masking indoor air concentrations attributable to subsurface contamination


Outline

Site Setting

Environmental Data


Role of Background



Role of BackgroundRange of Measured Site Indoor Air Concentrations versus Range of Literature Background Concentrations

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12 14Benzene Ethyl-benzene

Tetrachloro-ethene

Toluene Trichloro-ethene

Xylenes

80

Background concentrations from McHugh et al., 2004.

Site indoor air concentration range

Background indoor air concentration range


Role of Background

Measured indoor air concentrations at the Site consistent with literature background

Contributions from subsurface contamination vs. background at the Site cannot be conclusively apportioned

• Site-specific background data needed

Background indoor and outdoor air concentrations should be characterized to place risks in proper perspective

Challenges in background definition

• Variability in source and characteristics


Outline

Site Setting

Environmental Data


Role of Background




Building Area

• Current value (100 m2) only applicable for small residential buildings

• Need to develop AF graphs for other building sizes (medium, large) to expand applicability

Air Exchange Rate (AER)

• Current value (0.25 hr-1) also applicable at small residential buildings

• Users should have flexibility to use higher AERs – commercial buildings, warmer setting

vapor intrusion attenuation factors (afs) – measured vs . epa defaults a case study

Documents